In heat pump applications, the thermoelectric effect is used to provide for transfer of thermal energy from a cold source to a warm source by means of electrical energy supplied to thermoelectric elements made with thermoelectric material. For electricity generating applications, the thermal energy transferred from a warm source to a cold source enables the production of electrical energy at the terminals of the thermoelectric elements.
In both cases, the thermoelectric elements, referred to hereinafter for shorthand purposes as thermoelements, associated with heat exchange apparatus can be assembled according to two embodiments or modes.
According to a first mode of assembly, a known type of thermoelectric module can be used, these modules being built with a plurality of thermoelements comprising an integral number of thermoelectric couples (one N-type thermoelement and one P-type thermoelement) assembled together into a construction wherein they are connected electrically in series and disposed thermally in parallel. FIG. 1 shows a schematic representation of the assembly of such thermoelements. As can be seen in this figure, there are only two thermoelements, i.e., one couple with one N-type thermoelement 1 and one P-type thermoelement 2, connected together in electrical relationship by electrical connectors soldered on the surfaces of the thermoelements. There are two types of electrical connectors, those connectors denoted 11, which connect the surfaces of two thermoelements (electrically) in series and further connectors, denoted 12, which act as inlet and outlet for the electrical current into the module. To assure the continuity of the electrical path provided, it is necessary to electrically insulate the electrical connectors connecting the thermoelements from the associated heat exchangers. Generally, such insulation is obtained using ceramic sheets 13 in very close good contact with the electrical connectors. These sheets produce a significant thermal resistance between the thermoelements and the heat exchangers, thereby lowering the thermal efficiency of the modules. Moreover, the ceramic sheets are brittle and must be assembled with care and, because of this brittleness, it is not possible to exceed weak contact pressures (less than 1 MPa), thus resulting in significant thermal contact resistance.
Further, the dielectric rigidity of the ceramic sheets is somewhat low and the operation of the modules under high voltage conditions is not possible, because the electrical discharges which result break down the insulation.
To overcome these disadvantages, a second mode of assembly of the thermoelements has been used in association with heat exchangers. According to this second embodiment or mode, the heat exchangers provide by themselves the electrical current conduction. Accordingly, the thermal resistances, normally produced by the insulation used, are eliminated. It is noted that this mode is known as "direct transfer" and reference is made, for example, to U.S. Pat. No. 3,213,630 (Cecil J. Mole).
The integration of thermoelements with heat exchangers means that the sizes must be adjusted accordingly, so that the cross sectional area of the thermoelements must be adequate to provide sufficiently thermal power for optimal utilization of the heat exchangers. A large cross sectional area dictates the use of a high amperage current through the thermoelements and as a result, the electrical voltage produced is weak. Indeed, in order to produce a given thermal power level in the case of heat pump applications, or to generate a given level of electrical power in the case of electricity generating applications, the number of large cross sectional area integrated thermoelements must be lower than the number of elements of smaller cross sectional area thermoelements composing the modules used in the first mode of assembly discussed above.
Indeed, because the pumped thermal power, or the generated electrical power, is proportional to the (sum of the) areas of the thermoelements, the sum of the thermoelement areas of a thermoelectric module is, for the same output, equivalent to the area of the directly integrated thermoelements.
In the case of heat pump applications, a direct current electrical supply must be used which produces large current levels with low voltage, thus dictating an electrical supply which is heavy and large in size. In the case of electrical power generation, the current produced at a low voltage and relatively large amperages is not well suited for most applications.
For example, a thermoelectric assembly comprising 480 thermoelements with an individual area of 1.5 cm.sup.2 integrated to the heat exchangers, operates as a heat pump under fixed conditions with a voltage of 13 volts at a current of 200 amperes, while, in an electrical generating mode, for the same thermal conditions, the same assembly produces 30 amperes at 3.5 volts.